STRUCTURE OF THE ATOM

Matter has mass and takes up space. Atoms are basic building blocks
of matter, and cannot be chemically subdivided by ordinary means.

The word atom is derived from the Greek word atom which
means indivisible. The Greeks concluded that matter could be broken
down into particles to small to be seen. These particles were called
atoms

Atoms are composed of three type of particles: protons, neutrons,
and electron. Protons and neutrons are responsible for most of the atomic
mass e.g in a 150 person 149 lbs, 15 oz are protons and neutrons while
only 1 oz. is electrons. The mass of an electron is very small (9.108
X 10-28 grams).

Both the protons and neutrons reside in the nucleus. Protons have
a postive (+) charge, neutrons have no charge --they are neutral. Electrons
reside in orbitals around the nucleus. They have a negative charge (-).

It is the number of protons that determines the atomic number, e.g.,
H = 1. The number of protons in an element is constant (e.g., H=1, Ur=92)
but neutron number may vary, so mass number (protons + neutrons) may
vary.

The same element may contain varying numbers of neutrons; these forms
of an element are called isotopes. The chemical properties of isotopes
are the same, although the physical properties of some isotopes may
be different. Some isotopes are radioactive-meaning they "radiate"
energy as they decay to a more stable form, perhaps another element
half-life: time required for half of the atoms of an element to decay
into stable form. Another example is oxygen, with atomic number of 8
can have 8, 9, or 10 neutrons.

What are elements?

All matter is made up of elements which are fundamental substances
which cannot be broken down by chemical means. There are 92 elements
that occur naturally. The elements hydrogen, carbon, nitrogen and oxygen
are the elements that make up most living organisms. Some other elements
found in living organisms are: magnesium, calcium, phosphorus, sodium,
potassium.

By the late 1800's many elements had already been discovered. The scientist
Dmitri Mendeleev, a Russian chemist, proposed an arrangement of know
elements based on their atomic mass. The modern arrangement of the elements
is known as the Periodic Table of Elements and is arranged according
to the atomic number of elements.

Every atom would like
to have an electron configuration like a noble gases. In noble gases
the outer electron shell is complete. This makes the element chemically
inert. Helium is an example of a noble (inert) gas. It is not present
in organisms because it is not chemically reactive.

Historical Models of the atom

BOHR MODEL

Bohr model shows electrons circling the nucleus at different levels
or orbitals much like planets circle the sun. Electrons move from one
energy state to another but can only exist aft defineite energy levels.
The energy absorbed or released when electrons change states is in the
form of electromagnetic radiation.

THE WAVE MODEL AND QUANTUM THEORY

The Bohr model was only able to explain the very simplest atoms, like
hydrogen. Today's modern day theory is based on mathematics and the
properties of waves. The wave model forms the basis for the
Quantum Theory

. This theory gives the probability of locating electrons in a particular
location, unlike assuming electrons orbit the nucleus as in the Bohr
model.

How are electrons organized around the nucleus?

All atoms would like to attain electron configurations like noble
gases. That is, have completed outer shells. Atoms can form stable electron
configurations like noble gases

by:

losing electrons

sharing electrons

gaining electrons.

For a stable configuration each atom must fill its outer energy level.
In the case of noble gases that means eight electrons in the last
shell (with the exception of He which has two electrons).

Atoms that have 1, 2 or 3 electrons in their outer levels will tend
to lose them in interactions with atoms that have 5, 6 or 7 electrons
in their outer levels. Atoms that have 5, 6 or 7 electrons in their
outer levels will tend to gain electrons from atoms with 1, 2 or 3 electrons
in their outer levels. Atoms that have 4 electrons in the outer most
energy level will tend neither to totally lose nor totally gain electrons
during interactions.

The atomic orbitals of the hydrogen atom can be visualized as a cloud
around the nucleus. The orbital represents a probability of finding
the electron at a particular location. Darker regions signify a greater
probability. Shown below are the 1s (lowest orbital and the 2s orbital.

1s2s

Atomic orbitals do not always have the shape of a sphere. Higher orbitals
have very unusual shapes.

2px3px

These orbitals were prepared by Dr. Yue-Ling Wong from the University
of Florida for more images click
here.